The present invention relates to toner image pressure-fixation apparatus in general, and particularly to pressure-fixation apparatus including a pair of pinch rolls and a backup roll wherein at least the pair of pinch rolls is positioned with the axes of the rolls at an angle to each other.
In pressure-fixation apparatus for fixing a dry, pressure-fusible toner image onto a paper, it is known to use a pair of pinch rolls biased toward each other and disposed such that the axis of one roll extends at an angle relative to the axis of the other, so that any irregularity of the nip may be eliminated even when copying paper passes between these rolls.
However, when the cross angle is too large, the copying paper develops creases or wrinkles. In order to avoid such wrinkles in the copying paper, the cross angle must be kept small. When the cross angle is quite small, however, the rolls have to have large diameters to prevent a non-uniform nip by the bending of the rolls. However, if the rolls have larger diameters, not only the pressure-fixation apparatus weighs more but the loading pressure must be increased. This requires a larger pressure mechanism and a higher compression-resistant strength of the bearings of such rolls, attendant increase in strength of bearings, housing, etc., involving heavier weight and higher cost of manufacture.
In order to eliminate these difficulties, pressure-fixation apparatus including a so-called "three-roll system" has been previously proposed. In this system, a top roll and a bottom roll of a pair of pinch rolls are disposed parallel and horizontal without a cross angle between them and, in addition, a backup roll is disposed in a crossing contact relation to the bottom roll. The backup roll is biased resiliently upwards toward the bottom roll of the pair of pinch rolls by a pressure mechanism, while the bottom roll is butted against the top roll under pressure to form a nip along their contacting line through which a copy paper passes. As a flection of the bottom roll can be adjusted by the cross angle of the backup roll relative to the bottom roll, each of the bottom and the backup rolls may have a smaller diameter than the top roll and copying paper may be free from creases because of parallelism of the top roll with the bottom roll.
However, the three-roll system has the drawback of the offset phenomenon, i.e., toner having formed a toner image on a copying paper adheres to some extent onto the contacting roll at the time of pressure-fixing, and then this adhered toner returns onto the copying paper because or rotation of the roll, so as to degrade the resulting image.